Methods of synchronizing frequencies by performing correlation operation using a repetitive waveform of an orthogonal frequency division multiplexing (OFDM) packet are known (See, e.g., T. Sekiguchi, et al., “Development of Digital Wireless Transceiver for a MRI Coil with Clock Synchronization”, ISMRM 2009, and, P. H. Moose, “A technique for orthogonal frequency division multiplexing frequency offset correction,” IEEE Trans. Commun., Vol. 42, No. 10, pp. 2908-2914, October 1994). For example, in IEEE 802.11a being a wireless local area network (LAN) standard, a preamble which includes a repetitive waveform is allocated at the head of a packet. When the frequency used in a transmitter is completely the same as the frequency used in a receiver, the repetitive form is observed in the receiver. On the other hand, when the frequency used in the transmitter is shifted from the frequency used in the receiver, the receiver detects a phase difference which is proportional to the difference in the frequency. Therefore, it is possible to synchronize the frequencies of the transmitter and the receiver, by detecting a phase difference and performing correlation operation of a repetitive waveform.